This invention relates to supercritical fluid extraction.
In supercritical fluid extraction, an extraction vessel is held at a temperature above the critical point and is supplied with fluid at a pressure above the critical pressure. Under these conditions, the fluid within the extraction vessel is a supercritical fluid. In one type of apparatus for supercritical extraction, there is a specially constructed extraction vessel within a source of heat.
A prior art apparatus for supercritical extraction of this type is described by B. W. Wright, et. al., in ANAL. CHEM. 59, 38-44 (January 1987) using a glass-lined extraction chamber within a bolted stainless steel extraction vessel heated in an oven. This type of extraction apparatus has the disadvantages of: (1) requiring time consuming steps to open the pressurized extraction vessel before use to insert the sample and again to open it after use to remove the spent sample; and (2) under some circumstances, requiring the handling of a hot extraction vessel.
Prior art apparatuses for automatically changing samples are known. For example, Beckman Instruments, Inc. has produced a radioimmuno and a biogamma analyzer that incorporates a sample changer with an elevator mechanism that raises sample vials from a sample changer to a lead-shielded radiation counting chamber above the sample chamber. Also, a gamma series 300 unit manufactured by Beckman Instruments, Inc., automatically interposes a thick lead shutter that separates the sample vial and the counting chamber from the environment outside the counting chamber. These devices are described in Beckman Bulletin 7250 dated approximately 1972 or 1973. Another apparatus was produced by Micromedic Systems, a division of Rhom and Haas, called the Micromedic Concept 4. It is described in Bulletin M1515 dated 1976.
Two patents describing systems of this type are U.S. Pat. No. 3,257,561 to Packard et al issued Jun. 21, 1966, for RADIOACTIVITY LEVEL DETECTING APPARATUS FOR SAMPLES CARRIED BY PORTABLE TRAYS WITH TRANSFER AND INDEXING MEANS FOR THE TRAYS and U.S. Pat. No. 3,198,948 to Olsen issued Aug. 3, 1965, for APPARATUS FOR MEASURING ACTIVITY LEVELS OF RADIOACTIVE SAMPLES.
These devices are not suitable for handling the high temperature, high pressure fluid systems necessary for supercritical extraction.
To control the flow of supercritical fluids for an automatic supercritical fluid extractor, automatic valves are needed. Preferably, the valves are of economical construction and contain all the features of being able to handle high pressure, high temperature and have long life.
Prior art inexpensive automatically programmable valves do not contain all of these features and thus suffer from one of more of the disadvantages of short life, inability to handle high temperature, high cost or inability to handle high pressure.